What was found
LOST CABIN PRESSURE DESCENDING THROUGH APPROX FL 280. RECEIVED CABIN ALTITUDE WARNING MESSAGE. CABIN ALTITUDE SHOWED 10,000 & CLIMBING AT 500 FT/MIN. EMERGENCY DECLARED & DESCENDED RAPIDLY TO 10,000 FT, NORMAL RATE TO 5,000. WHILE LEVEL AT 5,000 FT, CABIN ALTITUDE EVENTUALLY SHOWED 5,000 FT. FOR DURATION OF FLT, CABIN ALTITUDE MATCHED ACFT ALTITUDE & CABIN DESCENT RATE MATCHED ACFT DESCENT RATE. EXECESSIVE CABIN PRESSURE LEAK PERFORMED IAW AMM 05-51-24, FOUND DURING TEST EQUIP COOLING OVERBOARD EXHAUST VALVE FULLY CLOSED POSITION & NO HAVE LEAKS. ALSO TEST PERORMED IAW AMM 21-58-05-5 OK. R&R BOTH CABIN PRESSURE CONTROLLERS IAW AMM 21-31-02-4, R & R DIFFERENTIAL PRESSURE SENSOR IAW 21-33-04-4.